Several known probe-based instruments monitor the interaction between a cantilever-based probe and a sample to obtain information concerning one or more characteristics of the sample. For example, scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically use a sharp tip and low forces to characterize the surface of a sample down to nanoscale dimensions. The term nanoscale as used for purposes of this invention refers to dimensions smaller than one micrometer. SPMs monitor the interaction between the sample and the probe tip. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular site on the sample, and a corresponding map of the site can be generated. Because of their resolution and versatility, SPMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
The probe of a typical SPM includes a very small cantilever which is fixed to a support at its base and which has a sharp probe tip extending from the opposite, free end. The probe tip is brought very near to or into contact with a surface of a sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector, often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, or some other deflection detector such as strain gauges, capacitance sensors, etc. The probe is scanned over a surface using a high resolution three axis scanner acting on the sample support and/or the probe, or a combination of both. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other surface property of the sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,226,801; and Elings et al. U.S. Pat. No. 5,412,980.
Different SPM probe tip shapes are used for a variety of applications. One type of tip shape that is commonly used for measuring the height of certain nanoscale features, for testing material properties (e.g. elastic modulus), or for manipulating very small objects is a pointed shape (e.g. parabolic) having a relatively simple profile. To image or measure surface features such as vertical sidewalls and undercut regions, and to take critical dimension (CD) measurements, SPMs utilize more complex probe tip shapes, such as boot-shaped or inverted mushroom-shaped probe tips, some of which may have one or more protrusions along the scanning direction.
SPMs may be designed to operate in a variety of modes, including modes for measuring, imaging, or otherwise inspecting a surface, and modes for measuring nanomechanical properties of a sample. Modes for inspecting a sample include contact mode and oscillating mode. In contact mode operation, the microscope typically scans the tip across the surface of the sample while keeping the force of the tip on the surface of the sample generally constant. This effect is accomplished by moving either the sample or the probe assembly generally perpendicular to the surface of the sample in response to sensed deflection of the cantilever as the probe is scanned horizontally across the surface. In this way, the data associated with this vertical motion can be stored and then used to construct an image of the inspected sample's surface, or to make certain measurements of selected surface features. Some SPMs can at least selectively operate in an oscillation mode of operation, in which the tip is oscillated at or near a resonant frequency of the cantilever of the probe. The amplitude or phase of this oscillation is affected by the tip-sample interaction, and any changes in the oscillation are sensed. These sensed changes are then monitored, stored, and processed into data that characterizes the sample. The collected data, in turn, may be plotted to image the sample surface, or analyzed to produce metrology data of certain dimensions of a feature on the sample's surface (such as, for example, a height of the feature). In applications for measuring nanomechanical properties of a sample, the probe is used to apply a mechanical stimulus to a sample, and monitor the resulting mechanical response of the sample. From this type of testing, material properties such as elastic or dynamic modulus may be measured. U.S. Pat. No. 7,055,378 describes nanomechanical applications for SPMs and associated techniques.
Notwithstanding the high resolution and accuracy capability of SPMs, the ultimate resolution of the data obtained by such probe-based instruments is limited by the physical characteristics of the tip of the probe itself. More particularly, there are limitations as to how small, and sharp, the tip can be made. For surface inspection applications, the tip shape is reflected in the acquired data, a problem that is exacerbated by the fact that SPMs often image very small (e.g., Angstrom scale) features. As a result, an error in the acquired data results and the corresponding accuracy of the surface image is significantly compromised. Similarly, for nanomechanical property measurement applications, the shape of the probe tip, i.e., its sharpness, substantially affects the force-deformation relationship.
For some applications, small variations in probe shape may be negligible. However, for many present-day applications, a high degree of accuracy and precision is required to resolve the features of interest on the sample surface, such that the tip shape variations must be accounted for. For instance, in the semiconductor fabrication industry, imaging features such as lines, trenches and vias with sub-nanometer accuracy and high precision is desired. These features may have dimensions in the range of tens of nanometers, and are continually getting smaller. With typical tip widths in the same size range, the shape of the probe tip introduces significant error in the data, which must then be corrected to accurately image the sample surface. The aforementioned challenges are further aggravated in situations where complex sample surface topography require a commensurate increase in tip shape complexity (and size) to image such surfaces.
Various reconstruction techniques are known for providing a correction based on characterizing the shape of the tip. Once the tip shape is measured or estimated, the variations in data measurements caused by the tip shape can then be “eroded” from the raw SPM sample image, for example, via mathematical morphology, thus yielding an improved “corrected” image. This is typically accomplished by removing or eroding the area (2-D; volume for 3-D) of the estimated probe tip shape for each position the probe tip occupies in the scan. In another known and widely used technique, a simple subtraction of the tip-width in the scan direction can provide improved reconstructed images and critical dimension measurements. Various known morphological techniques are described, for example, in J. S. Villarrubia, Algorithms for Scanned Probe Microscope Image Simulation, Surface Reconstruction, and Tip Estimation, J. Res. Natl. Inst. Stand. Technol. 102, 425 (1997). Other techniques that account for the probe tip's interaction point with the sample surface for more accurate correction of measurements are described, for example, in U.S. Pat. No. 6,810,354.
In order for these techniques to provide an accurate correction, the dimensions of the CD tip must be determined with high accuracy. The way in which this is typically accomplished is by scanning a tip characterizer structure or set of structures that have well-known dimensions with the probe tip. Characterizer structures include, for example, a silicon nanoedge, a nanostructure having a width reference structure, or a cavity-type structure having acute angled corners at its entrance. Because the dimensions of the characterizer structure are known or at least very closely approximated, various dimensions of the probe tip can be obtained during a probe tip characterization procedure. In a typical probe tip characterization procedure, the probe is scanned over the characterizer of known dimensions to produce an image data profile that includes distortion introduced by the shape of the probe tip. The image data profile is then analyzed to identify the tip distortion and to generate a probe tip correction factor based thereupon, which is applied to correct the raw measurements of actual samples.
Probe tip wear is another challenge that is becoming of greater concern as SPM applications continue to demand greater resolution. Probe wear occurs when probe tips interact with the samples in the course of conducting measurements. Material can be lost from, or picked up by, the probe tips, causing changes in the size and shape of the tips. Different types of probes (in terms of shape or materials) have different wear characteristics, and even probes of the same type can wear differently for a variety of reasons. Indeed, the same probe can experience different wear trends depending on the nature of the samples being scanned by the probe, the corresponding diverse types of interaction between the probe tip and the samples, and other changing circumstances.
A standard technique for dealing with the issue of probe wear requires that the probe tip be re-characterized over the course of measuring samples as described above. The process of characterization is time consuming, requiring removing the probe tip from the sample being measured and aligning the probe with a characterizer, then running a characterization routine to scan the characterizer structure. Sometimes, such as for critical dimension probe tips, more than one characterizer structure is needed to characterize all of the necessary probe tip dimensions. The probe is then re-aligned with the sample for further measurement. The re-characterization procedure can take up to 10 minutes or more, which is extremely costly in terms of capacity for high volume manufacturing applications such as semiconductor fabrication. The time and frequency required for performing tip characterization can significantly affect the cost of ownership of the instrument, and the cost and speed measurements per wafer. Thus, probe wear presents a throughput problem in addition to measurement performance-related problems. Moreover, the occurrence of probe wear creates a design trade-off between measurement performance and throughput in which applying known techniques to improve measurement performance compromises throughput, and vice versa.
Known techniques for managing probe wear based solely on probe re-characterization measurements are becoming impracticable as any additional process bottlenecks present substantial costs for SPM operators, particularly in high volume production processes. In view of these and other related challenges concerning probe tip wear, a solution is needed for improving nanoscale measurement performance and throughput.